Pneumatic materials conveying system



Feb. 11, 1964 1.. D. MCDONALD PNEUMATIC MATERIALS CONVEYING SYSTEM 2Sheets-Sheet 1 Filed Nov. 6, 1961 INVENTOR. law/72044 fl Mia/74b .4TTORNEYS.

INVENTOR.

A77'ORNEK5.

Feb. 11, 1964 D. MCDONALD PNEUMATIC MATERIALS CONVEYING SYSTEM 2Sheets-Sheet 2 Filed NOV. 6, 1961 Law/ante Q/WeWa/m/d BY Z Mu?! UnitedStates Patent 3,329,979 PNEUMATIC MATERHALS NVEYHNG SYSTEM Lawrence D.McDonald, 455d Main St., Kansas City, Mo. Filed Nov. 6, 19M, Ser. No.l',63 9 Claims. (Q1. 3ll235) This invention relates generally topneumatic materials conveying systems, and refers more particularly toimprovements therein for better and more eilicient operaion of same.

Pneumatic conveying of finely divided materials, for example flour,cement and the like, has become widely accepted. In particular, it isbeing used quite successfully in flour mills, large bakeries and thelike. However, there are many problems, both as to first cost and as tooperation, which still remain to be overcome.

In most plants where such systems are used, it is conventional to have aplurality of what may best be termed sub-systems, each of which has itsown air pump and control. There is no interrelation between subsub-systems, other than perhaps they may be connected with a mastercontrol panel. in other words, the air from any given pump is confinedto the specific sub-system for which it is designed.

Each of the sub-systems may include various bins which can be cut intoor out of the sub-system as needed, as well as various outlets for thematerial. However, in designing the system, the air pump therefor mustbe selected on the basis of one which will supply sufficient air toeffectively power the sub-system at its fully loaded condition. Sincemost sub-systems are ope-rated at many times at less than full load,that is, with certain component parts cut out, this means that there isoften considerable wastage of power. Moreover, with separatesub-systems, each sub-system requires its own pump and if there are anumber of sub-systems, the first costs are quite hi r.

One of the principal objectives of the present invention is to provide apneumatic conveying system which is so constructed and controlled thateach sub-system derives its air supply from a single air source, namely,a header or air main which has connected with it in common the necessarynumber of pumps to meet the maXimum load conditions for the system as awhole. Each sub-system is supplied by a branch line from the commonheader; however, through special control means the air supplied to eachbranch line is held to a velocity which is selected as optimum for theconveying conditions to be met, and this is true despite increase ordecrease in the resistance to air flow through the sub-system. Statedotherwise, each branch line is so controlled that it will receive airfrom the header as required by the conveying conditions, and no more.

Since it is seldom, if ever, that all branch lines will be insimultaneous operation, I am thus able to limit the number of air pumpsto less than that which would be required for separate sub-systems, thussaving greatly on the cost of the over-all system. Moreover, byproviding for control in the individual branch lines of the air flowtherethrough in response to demand, the air supplied by the pumpingsystem is efficiently utilized where actually needed. It is a feature ofthe invention in this respect that the control means operates to preventair robbing through empty lines at the expense f loaded lines.

A further important object of the invention is to provide a multipleline pneumatic conveying system in which the air is supplied from acentral source, yet in which the back pressure on the air pumpingequipment is automatically controlled to a minimum value required tomeet the air flow requirements of the system as a whole.

Since back pressure determines the load on the pumping equipment,through this arrangement I have been able to eifcct a great saving inoperating power.

till another object of the invention is to provide a system of thecharacter described in which multiple pumps are connected with theheader, but in which the pumps individually are automatically cut intoand out of operation in accordance with the air demand in the system. Afeature of the invention in this respect resides in the manner ofcontrolling the pumps and the means for cutting in or taking out pumpsunder changing conditions.

Yet another object of the invention is to provide a pneumatic conveyingsystem which, because of the use of a common air source for allconveying lines, lends itself to highly flexible operation and makespossible expansion of the system to include additional lines at lowcost.

A further object of the invention is to provide unique means forcontrolling the air velocity through the conveying lines to maintain itsubstantially constant despite changes in static pressure, which meansis capable of instant operation in response to changing flowcharacteristics in the conveying line.

Other and further objects of the invention together with the features ofnovelty will appear in the course of the following description.

in the accompanying drawings, which form apart of the instantspecification and are to be read in conjunction therewith, and in whichlike reference numerals indicate like parts in the various views:

FIG. 1 is a schematic diagram showing a system constructed in accordancewith a preferred embodiment of the invention; and

FIG. 2 is an enlarged detail illustration, still principally schematic,showing the differential pressure comparator, the air relay and aircontrol valve for one of the branch lines of the preferred embodiment.

The principles of the invention will be described as applied in anillustrative pneumatic flour conveying system involving two positivedisplacement air pumps 10 and ii. The pumps 1% and ill have theirdischarge outlets connected with a common header 12 by lines 19a and11a, respecitvely. in the example being given the header l2 feeds air tothe branch lines 14, 14 and 14'. To facilitate the description, andsince the components associated with each branch line are in substanceidentical, detailed amplification will be confined to the lefthandbranch line 114- with the understanding that the same reference numeralsfollowed by a prime identify like components on the branch line 14, andthat branch '14" has the same components (although not shown).

As shown, the branch line 14 has interposed therein a variablethrottling valve 15 which is operated by a pressure responsive actuatingmechanism to later to be described. The air passed by the valve 15 isdirected to the input side of a conventional air lock rotary materialfeeder 17. The feeder i7 is located at the bottom of a storage bin orhopper 18. The feeder includes the drive motor 19, which, as is known bythose skilled in this. art, drives the rotor of the feeder to delivermaterial to an air pickup position from whence it is carried into thedischarge conveying line 20.

Returning now to the pumps ill and 11 and considering first pump Ill, itwill be seen that this is powered by an electric motor 21 which isadapted to be connected with the power lines L L and L through themagnetic starter 22,. Conductors 23, 24 and 25 are connectedrespectively with the lines and have the normally open contacts 26, 2'7and 23 interposed therein. The starter solenoid 29 operates the latterin the usual fashion. The starter solenoid is included in a circuitbetween lines L and L which includes conductor 31, main throw switch 32,and conductors 33, 34 35, 36 and 24. Obviously, upon closing of switch32 solenoid 29 will be energized, thus closing the starter contacts andcompleting the circuit to motor 21 to start the pump The second pump 11has its drive motor 37 which is adapted to be connected to lines L L andL through conductors 38, 39 and iii, and the magnetic starter 41. Thestarter solenoid 4-2 of this pump is in a circuit between lines L and Lwhich includes conductor 35:, switch 32, conductors 33, 43, no mallyopen contact 44 of relay i5, conductor 46, the normally open contact 47having the pressure actuating mechanism :8, conductors 49, and 51 havinginterposed therein the normally closed contact 52 with a pressureactuating mechanism 53, and conductor 39. It Willbe observed that theclosing of switch 32 serves to arm the circuit to the second startersolenoid 42 by energizing the relay 45. The latter is connected inparallel with the first starter solenoid 2% by conductors 54- and 55.However, the circuit to starter solenoid 42 is not completed until thecontact 47 is thereafter closed. 'I hus, pump 11 will not be started atthe same time as pump 10, the reason for which will subsequently bebrought out.

Turning again to the air valve and its associated component, theoperation of this valve is preferably controlled by a difierentialpressure regulator 56 having an relay 57. The output line 58 from therelay leads to the pressure sensitive valve actuating mechanism 16. Theinput line to the air relay is indicated at 59, this being a branch froma main air line as having a constant pressure outlet valve 61 and acontrol valve 62. The line 6i? leads from any suitable source ofcompressed air (not shown), such as a compressor, and is designed tomaintain a supply of air to the operating system at from 15 psi. top.s.1.

Referring now to FIG. 2 and describing in more detail the nature andmanner of operation of air valve 15 and its control components, providedinside conduit 14, preferably downstream of the valve 35, is a iitottube 63 having the nose port 64 land the static ports 65. The pressureat port 64 is communicated by line 66 to a chamber 67 formed in ahousing 68. The static pressure measured at ports 65 is passed by line69 to another chamber 7 ii in the housing. The chambers 67 and 70 areseparated by a diaphragm 71 which carries a post '72 extending outwardlythrough the wall of the housing, a suitable seal being provided toprevent pressure leakage from chamber 7% while still permitting up anddown movement of the post in response to flexing of the diaphnagm underpressure diiierentials in the chambers 67 and 7d. The post 7 2 bears atits outer end against a lever 73 pivoted as at 74L to a stationarysupport. The free end of lever 7 3 controls a leak port 7 5 forming apart of the air relay 57.

It will be understood that for the purpose of simplifying thedescription I have shown only the principal elements of a typicaldifferential pressure regulator 56. Such devices are well known andcommercially available. An eminently suitable commercial deviceoperating on these principles is the model R=2 sold by Johnson ServiceCompany of Milwaukee, Wisconsin.

The air relay 57 is of the direct acting gradual type and is alsocommercially available from the same company under model number Rl. InFIG. 2 such a device is shown in simplified form. Air pressure from thepressure line 6 3 is fed by branch line 59 into a first chamber 76having the outlet 77 controlled by the normally closed valve 7 8. Outlet77 leads to a second chamber '79 which is in communication with theoutput line 58 to the valve regulator 16. 'The chamber 79 communicateswith an adjacent chamber 3t) through the center hole in an exhaust seat81 and the ports 32 of a spacer 83.

As will be noted, chamber is open to the atmosphere through the aperture34. Therefore, under normal conditions (before start-up of the system),atmospheric pressure exists in the line 53.

The air pressure from line 59 is also supplied to a chamber 85 throughpassageway 86 and restrictor 87. As will be evident, the pressure inthis 35 is determined by the position of the lever 7 3 relative to theleak port 75. Under dead air conditions in conduit 14, i.e., no airmotion, the pressures in chambers 67 and 7% of the differential pressureregulator will be equal. At this time the lever 73 is in its closestposition to the leak port, which is the position illustrated in solidlines in FIG. 2.

It may be helpful to go briefly into the operation oi the pressurediiierential regulator and relay at this point. When the entire systeminoperation, air is available to the relay through line 59 at apreselected pressure, say 15-28 psi. Immediately the pressure isapplied, it builds up in chamber 85 of the relay and :forces diaphnagm85a to the right. The movement of the latter is transmitted throughspacer 83 to diaphragm title and the exhaust valve seat 851. The latteris seated against the valve 7%, thus closing chamber '79 to theatmosphere. A continued increase in pressure in chamber causes furthermove- .cnt of exhaust seat 81 to the right, forcing the valve 78 openand allowing pressurized air from line 59 to enter chamber 79 and theline 5%. The pressure in chamber 79 rises until this pressure, actingagainst the diaphragm 85a, forces exhaust seat 81 to the left and allowsthe valve '73 to close.

The relay is now in a balanced position with the force exerted by thepressure in chamber 353 acting on diaphragm 85a exactly equal to theforce exerted by the pressure in line 53 acting on diaphragm 89a, andsince the leak port is at its most closed position, this means a maximumpressure is being transmitted through line 58 to valve actuatingmechanism 16.

Obviously, upward movement of lever 73 will result in a reduction inpressure in chamber 85. The force then exerted by the pressure in line58, plus the force of spiral spring 88, are greater than the force ofthe pressure against diaphragm 8'7, and exhaust seat n1 is thereforemoved away from the valve '78. Air from line 58 thus escapes toatmosphere through the center hole in the exhaust seat and exhaust hole84. When the pressure in line 58 is reduced to the point Where theforces are again balanced, exhaust seat 31 closes against valve 7 8.

From the foregoing description it is seen that for every value ofpressure existing in chamber 85, there is required a definite value ofpressure in line 5% in order for the forces operating the valvemechanism to be in balance. Any other pressure in line E8 would resultin an opening either at exhaust seat 31 or outlet 77 which would causethe pressure in line 58 to change in the directions required to bringabout the necessary balance.

Obviously, it is the difierential between the total pressure measured atport d4 of the Pitot tube and the static pressure measured at port d5which determines the pressure in line 58 at any given time. Thisdifferential is actually a measurement of the air velocity through theconduit 1 14. With no air motion, lever 73 is in the position shown insolid lines in FIG. 2, which, as earlier noted, corresponds to themaximum pressure in line 58. As the velocity past the Pitot tubeincreases, the total pressure will increase and lever 73 will be lifted,thus proportionately reducing the pressure in line 58. This in turnoperates to change the condition in the valve 15.

The valve 15 and its pressure actuating mechanism 16 are conventional inconstruction. The valve member is biased toward the closed position bythe spring @1. Line 58 is connected with the diaphragm chamber 5 2. Thediaphragm 93 serves to shift the stem 94 of the valve thus to vary theamount of air that will be passed by the valve downstream toward andpast the Pitot tube 63. It will be evident from the description that haspre ceded that when the control air pressure is present in line 5h tothe relay 5'7, the first tendency of the pressure actuating mechanism isto shift the valve body 99 to the full open position. The position ofthe body 9%) thereafter will be dependent on the velocity head inconduit 14- downstream of the valve.

In designing a complete system I prefer to so relate the pressureregulator 56, relay 57 and valve actuating mechanism 16 so that at notime during operation will the valve body 9% completely close againstits seat. In other words, I desire to maintain a substantially constantvelocity of air through conduit 14 downstream of the valve under allconditions, including the condition in which bin 18 is empty and conduit20 has no resistance in it. For purposes of illustration, a rate of4,000 feet per minute will be used. Obviously the Pitot tube 63 gives ameans of determining velocity despite variations in static pressure andthis measurement of velocity is in turn utilized to control the valve 15so that the design velocity will be maintained despite changing staticpressures.

Going back again to FIG. 1, it will be seen that I have provided a thirddifferential pressure regulator and relay ltlil which has the air inputline 101 from the pressure line 60. The unit 100 is in all respects likethat of the unit 56. The output line from the relay is indicated at 192and this line leads to the pressure actuating mechanism 53 for contact52, the pressure actuating mechanism 43 for contact 47, and a pressureactuating mechanism 104 for a variable pressure relief valve 105. Thelatter components are similar in construction and operation to the mainair valve 15 and its operating mechanism 16. The pressure relief valveN5 is mounted in an exhaust line 106 connected with the header 12.

The pressure in line 102 is controlled by the unit 100 in response todifferentials in static pressure between the header l2 and the highestof the static pressures in line 14, 14 and 14". The static pressure inthe header is transmitted to one side of the regulator 1% through line107. The static pressure from each of branch lines 14-, 14' and 14/ istaken from a point downstream of the valve by lines 108, 1% and 163"respectively, to a pressure selector it of conventional construction.The pressure selector selects the highest of the pressures in lines 108,103 and i553" and passes it to the other side of regulator 10% throughline 110.

It will be apparent that the pressure in line 102 from the pressureregulator 100 is thus a direct function of the static pressuredifferential between the header 12 and the highest pressure in line 14,14', 14 on the downstream side of valve l5, l5, 15'. The pressureactuating echanism 104 of the variable relief valve 105 is preset sothat the valve will maintain a preselected positive differential betweenthe header and highest branch pressure between certain maximum andminimum limits. In other words, valve 195 will be at its full closedposition when the differential drops below the preselected minimum andwill be full open when the maximum differential is reached. In thevariable range the valve should operate to maintain a low positivedifferential, say 1 psi. The actuating mechanism 53 for contact 52 isset to cause opening of contact 52 at the preselected maximumdifferential. On the other hand the actuating mechanism 48 for contact47 is so set that contact 47 will close when at the preselected minimumdifferential.

Operation In starting up the system switch 32 is closed, thus startingpump 10 which supplies air to the header 12 and the branch lines 14, i4and 14-". Air valve 62 is also opened to arm the control system withcontrol air pressure through line 60. It will be assumed that at theoutset none of the feeders is being operated so that in effect there islittle resistance to flow through the branches i4, 14- and 14" and theirconveying conduits. I11 this condition valves l5, 15' will be in themost restricted condition as will also be true with the similar valve(not shown) in line 14-", with the static pressure on the downstreamside of the valves very low and the valves open only enough to supplythe preselected velocity head through the conveying conduits.

Because of the restriction at the valves 15, 15' the static pressure inthe header will be relatively high and the pressure differentialmeasured at regulator will also be high. Pressure relief valve 1% willbe full open, contact 47 still open, and contact 52 closed since pump 10cannot inherently supply enough air to result in opening of thiscontact.

Assume now that feeder 17 is energized and material starts feeding intoline 20, thus increasing the resistance to air flow and increasing thestatic pressure in line 14 downstream of the valve 15. The increase instatic pressure results in an initial reduction in the air velocity,which is reflected at Pitot 63. The change at Pitot 63 immeditelyactuates the regulator and relay 56 to change the setting of valve 15 toopen it further. Through the regulator, valve 15 will be instantaneouslyadjusted to produce the design velocity despite the increase in staticpressure. As will be evident as the pressure and velocity at the Pitottube change, valve 15 will be operated to further restrict or increasethe valve opening as the situation may require.

Air robbing through the as yet empty lines 14 and 14" is prevented bytheir valves, which remain in the most restricted condition because ofthe low static pressure below the valves. Therefore, the header will beable to supply enough air to line 20 to effect satisfactory conveyingdespite the fact that the other branches are not loaded. Moreover,relief valve 1% will begin to relieve the header if the static pressurein the header rises enough above the static below valve 15.

It is assumed that the over-all system is so designed that the capacityof pump 10 is such that it could adequately supply enough air to effectnormal material conveying through two of the branch lines such asbranches is and 14. However, it is not able by itself to also supplyenough air for this purpose in the third branch 14". Accordingly, solong as the third branch remains inactive and so long as normalconveying pressures are present in lines 20 and 20', pump 10 alone willbe enough for the system.

If, however, the air demand from the over-all conveying system becomesgreat enough that the differential pressure measured by regulator 100drops to the preselected minimum, the contact 47 in the starting circuitto pump 11 will be closed by the pressure actuating mechanism 48. Sincethis circuit has already been armed by closing of contact 44 at the timeof starting the pump 10, starter 41 will now be closed, bringing pump 11into operation.

Since pump ll supplies additional air, the diiferential at regulatorltltl again increases. While contact 47 will immediately reopen due tothe diiferential rising above the selected minimum, the starter remainsenergized through the locking circuit from conductor 39 through contact52 and conductors 50 and 51.

The second pump 11 will remain operating so long as pressure in line 1Mdoes not exceed the predetermined value which will cause pressuremechanism 53 to open contact 52;. However, if the load on the systemdecreases to the point where the pressure in control line 192 risesabove said value then contact 52 will be opened, thus breaking thelocking circuit across starter 41 and stopping pump 11. Thereafter pump10 will operate alone to supply air to the system until such time as theconveying air demand rises above what that pump can effectively supply,again bringing pump 11 into play, as previously described.

It is important to note that the pressure differential at regulatorTilt) is always measured between the highest conveying line staticpressure and the static in the header. The selector 10$ passes only thehighest of the conveying line pressures, so the demand of the system isalways based on the most loaded conveyor conduit. Moreover, through thepneumatic control system for the valves I am able to achieve almostinstantaneous response to any change in conditions in the lines, thusmaintaining continuous flow at all points and avoiding any intervals ofair robbing through unloaded lines.

By virtue of the arrangement employed the load on the pump meters 21 and37 is maintained substantially at that which is required for effectiveoperation of the system, thus effecting a great saving in power ascompared with systems of which I am presently aware. The relief valve 1%serves to relieve the back pressure whenever substantially more thansufficient air for conveying is available. The ability of the system toautomatically cut pump 11 into and out of use likewise serves to holdthe costs of operation to a minimum since this pump is used only whenneeded. 7 It will be evident that the system can be expanded to includemore pumps and more lines by simply connecting each additional pumpmotor into the preceding one in the same relationship as between pumps10 and 11. The pressure regulators and pressure switches associated withthe additional pumps would be set at the values necessary to cut in eachadditional pump as the system demands.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forthtogether with other advantages which are obvious and which are inherentto the ructure. It will be understood that certain features andsubcombinations are of utility and may be employed without reference toother features and subcombinations. This is contemplated by and iswithin the scope of the claims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

1. In a pneumatic conveying system, a header, a plurality of air pumpsconnected with said header and operable, when energized, to supply airto said header, a plurality of branch discharge lines each connectedwith said header, each said line having interposed therein a variablethrottling valve, velocity sensing means located in each line, controlmeans for each valve operably connected with the sensing means in theline containing such valve, each said control means operable to vary thevalve in a direction to tend to maintain the velocity through said lineconstant despite changes in static pressure in the line, and meansoperable to vary the static pressure in said header at a predetermineddifferential relationship with the highest of the static pressures inthe respective lines.

2. In a pneumatic conveying system, a header, a plurality of air pumpsconnected lWlth said header and operable, when energized, to supply airto said header, a plurality of branch discharge lines each connectedwith said header, each said line having intenposed therein a variablethrottling valve, velocity sensing means located in each line, controlmeans for each valve operably connected with the sensing means in theline containing such valve, each said control means operable to vary thevalve in a direction to tend to maintain the velocity through said lineconstant despite changes in static pressure in the line, and meansoperable to viary the number of pumps in simultaneous operationresponsive to the static pressure differential between said header andthe highest of the static pressures in said lines.

3. In a pneumatic conveying system, a header, first and econd air pumpseach connected to discharge into said header, first and second motorsrespectively drivingly connected with said pumps, a plurality of branchlineseach connected with and leading from said header, each line havingfeeder means for selectively introducing material to be conveyedtherein, a variable throttling valve in each line ahead of said feedermeans, velocity sensing means in each line operable to measure the airvelocity therethrough, valve control mechanism connected with andoperated responsive to said velocity sensing means to vary said valve inresponse to changes in velocity in the line from a predeterminedconstant velocity, and pump control means operable to energize anddeenergize said second pump motor in response to selected minimum andmaximum static pressure diiierentials between said header and thehighest static pressure in said respective lines.

4. In a (pneumatic conveying system as in claim 3, static pressurerelief means connected with said header and operable to tend to maintainthe static pressure in said header at a selected limited differentialwith respect to the highest of the static pressures in said branchlines.

5. in a pneumatic conveying system as in claim 4, said relief meansincluding an outlet from said header, a variable throttling valve saidoutlet, static pressure sensing means connected respectively with saidlines and said header, and regulator means for said valve in said outletconnected with said (sensing means and actuated responsive thereto.

6. in a pneumatic conveying system, a header, first and second air pumpseach connected to discharge into said header, first and second motorsdrivin-gly connected with said pump, starter circuits for said motorsand operable to selectively energize same, means operable upon closingof the starter circuit to said first motor to arm, but not close, thestarter circuit to the second motor, a plurality of branch lines eachconnected with and leading from said header, each line having feedermeans for selectively introducing material to be conveyed therein, a

variable throttling valve in each line ahead of said feeder means, airvelocity sensing means in each line operable to measure the air velocitytherethrough, valve control mechanism connected with and operatedresponsive to said velocity sensing means to vary said valve in responseto changes in velocity in the line from a predetermined constantvelocity, and pump motor control means operable to close and open thestarter circuit to said second motor in response to selected minimum andmaximum static pressure diiierentials between said header and thehighest static pressure in said respective lines.

7. In a system as in claim 6, said pump motor control means includingpressure sensitive switches interposed in said starter circuit to saidsecond motor and respectively sensitive to said minimum and maximumdifferentials.

8. In a system as in claim 7, said switch responsive to said minimumdifferential being normally open and said switch responsive to saidmaximum differential normally closed.

9. In a system as in claim 8, said normally closed switch operable tomaintain said starter circuit to said second motor closed followingclosing of said normally open switch until said maximum pressuredifierential is attained.

References Cited in the file of this patent UNITED STATES PATENTS2,404,937 Anderson July 30, 1946 2,665,707 Stover Ian. 12, 19542,726,122 Hagerbanmer Dec. 6, 1955 2,770,584 Ray Nov. 13, 1956 2,884,940Garrie May 5, 1959 3,002,521 Greenlees Oct. 3, 1961 3,005,462 HillmanOct. 24, 1961

1. IN A PNEUMATIC CONVEYING SYSTEM, A HEADER, A PLURALITY OF AIR PUMPSCONNECTED WITH SAID HEADER AND OPERABLE, WHEN ENERGIZED, TO SUPPLY AIRTO SAID HEADER, A PLURALITY OF BRANCH DISCHARGE LINES EACH CONNECTEDWITH SAID HEADER, EACH SAID LINE HAVING INTERPOSED THEREIN A VARIABLETHROTTLING VALVE, VELOCITY SENSING MEANS LOCATED IN EACH LINE, CONTROLMEANS FOR EACH VALVE OPERABLY CONNECTED WITH THE SENSING MEANS IN THELINE CONTAINING SUCH VALVE, EACH SAID CONTROL MEANS OPERABLE TO VARY THEVALVE IN A DIRECTION TO TEND TO MAINTAIN THE VELOCITY THROUGH SAID LINECONSTANT DESPITE CHANGES IN STATIC PRESSURE IN THE LINE, AND MEANSOPERABLE TO VARY THE STATIC PRESSURE IN SAID HEADER AT A PREDETERMINEDDIFFERENTIAL RELATIONSHIP WITH THE HIGHEST OF THE STATIC PRESSURES INTHE RESPECTIVE LINES.